This invention relates to a digital transmission system comprising a transmitter coupled to a receiver via a channel, the receiver comprising a detector for determining detected symbols at detection instants from a detection signal derived from the input signal of the receiver, timing means for determining a timing error signal by correlation of a first auxiliary signal derived from the detected symbols with a second auxiliary signal derived from the input signal of the receiver, and for adapting the detection instants in response to the timing error signal.
The invention likewise relates to a receiver for such a transmission system.
A transmission system as defined in the opening paragraph is known from the journal article "Timing Recovery in Digital Synchronous Data Receivers" by K. H. Mueller and M. Muller in IEEE Transactions on Communications, Vol. COM-24, No. 5, May 1976.
Transmission systems of this type may be used, for example, for transferring data signals via a transmission medium, for example, a cable or radio link, or for reconstituting data signals coming from a magnetic tape or disc.
When data symbols are transferred via a transmission medium or data symbols are stored on a recording medium respectively, the symbols to be transmitted or recorded respectively, are converted by coding means into data pulses which are applied to the transmission medium or recording medium further to be referenced by the term channel.
The output signal of the channel is applied to the input of the receiver. A detection signal is derived from the input signal of the receiver. This may be effected, for example, by an adaptive equalizer and/or an adaptive echo canceller. It is likewise conceivable that the detection signal is determined from the input signal of the receiver by a fixed filter.
At suitable detection instants the detector determines a series of detected symbols from the detection signal. The detector may comprise a simple bit-by-bit detector, but it is alternatively conceivable that the detector comprises a so-called Maximum Likelihood Sequence Estimation detector such as, for example, a Viterbi detector.
If the detection instants differ from the optimum detection instants, the contribution to the detection signal by different symbols from the symbol to be detected will generally increase, which will reduce the reliability of the decision made about the value of the detected symbol. This contribution is called intersymbol interference.
In order to avoid this reduction of the reliability of the decision about the correct symbol value, prior-art transmission system comprises timing means for deriving suitable detection instants from the input signal of the receiver.
In the prior-art system a clock signal indicating the optimum detection instants is derived from the input signal of the receiver and the value of the detected symbols.
A first method of determining the detection instants, which method is known from said journal article, is determining a timing error signal by correlating a first auxiliary signal, i.e. the detected symbol value delayed by a symbol interval, with a second auxiliary signal, i.e. the detection signal. The drawback of such a simple method is that a considerable amount of timing information present in the input signal of the receiver is not utilized. As a result, the detection instants show a certain jitter which leads to a sub-optimum detection.
To reduce the jitter during the detection instants, said journal article discloses timing means which utilize more first auxiliary signals which are derived each from a number of symbol values detected at different detection instants. Each of these auxiliary signals is correlated by its own correlator with an associated second auxiliary signal derived from the input signal, while a control signal for a controllable oscillator to adapt the detection instants is obtained by adding together the output signals of the correlators.